1. Field of the Invention
The present invention relates to a member for electrophotography such as a fixing member or pressurizing member to be used in a heat fixing device for an electrophotographic apparatus such as a copying machine or a printer.
2. Description of the Related Art
In an electrophotographic apparatus, a heating device including a heating member and a pressurizing member arranged so as to face the heating member has been used as a heating device for fixing an unfixed toner image formed on a recording material onto the recording material.
In a heat fixing device to be used in an electrophotographic system, a pair of heated rotating members as members for electrophotography like a roller and a roller, a film and a roller, or a belt and a roller have been generally brought into press contact with each other. Then, a recording medium holding an image formed with unfixed toner is introduced into a press-contacting site formed between the rotating members and heated. Thus, the toner is melted and hence the image is fixed onto the recording medium.
The members for electrophotography for the heat fixing device include a fixing member and a pressurizing member. Herein, a member that comes into contact with the unfixed toner image held on the recording medium to heat the unfixed toner is referred to as “fixing member”. The fixing members are classified into, for example, a fixing roller, a fixing film, and a fixing belt depending on their forms. Meanwhile, a member that is arranged so as to face the fixing member and forms a fixing nip together with the fixing member is referred to as “pressurizing member”. The pressurizing members are classified into, for example, a pressurizing roller, a pressurizing film, and a pressurizing belt depending on their forms.
Incidentally, a problem in the case where an attempt is made to cause the heat fixing device to correspond to recording materials (such as paper) of various sizes is an increase in temperature of the region of a member for electrophotography with which a small-size recording material (such as A4 size paper) does not come into contact. Such region is specifically, for example, an end region in the width direction of the fixing member.
That is, when paper whose width in a longitudinal direction is relatively small (hereinafter sometimes referred to as “small-size paper”) as compared to paper of the largest size onto which an image can be fixed by the heat fixing device (hereinafter sometimes referred to as “full-size paper”) is continuously passed through the fixing nip like FIG. 6, unlike a region where the small-size paper passes (longitudinal direction width W1), the surface of a member for electrophotography in a region where the small-size paper does not pass (longitudinal direction width W2) is not deprived of heat by the small-size paper.
As described above, in the region in the member for electrophotography with which the small-size paper does not come into contact (the region is hereinafter sometimes referred to as “non-paper passing portion”), heat from the fixing member is not stolen by a recording material or toner on the recording material, and hence the temperature of the non-paper passing portion of the member for electrophotography increases. The phenomenon is hereinafter sometimes referred to as “non-paper passing portion temperature increase.”
Such phenomenon is more likely to occur as the speed at which a printer outputs an image (process speed) increases. This is because of the following reason. The time period for which a recording material passes the nip shortens in association with the increase in image output speed, and hence sufficient heat needs to be transferred to a toner image within an additionally short time period. To this end, the temperature of the fixing member needs to be additionally increased.
The member for electrophotography to be used in such heat fixing device generally has a construction obtained by forming an elastic layer containing a silicone rubber cured product on a substrate formed of, for example, a metal or a heat-resistant resin, and bonding a fluorine resin tube onto the layer through a silicone rubber adhesive (Japanese Patent Application Laid-Open No. 2008-176300).
In addition, studies made by the inventors of the present invention have found the following new problem. When a heat fixing device using a member for electrophotography having the construction is operated over a long time period, the hardness of the non-paper passing portion of the member for electrophotography increases, and hence a difference in hardness between the non-paper passing portion and a portion except the non-paper passing portion enlarges.
When a toner image formed on full-size paper is fixed by using the member for electrophotography in which a large hardness difference has occurred between the non-paper passing portion and the portion except the non-paper passing portion as a fixing member, toner with which the non-paper passing portion of the fixing member relatively increased in hardness comes into contact is squashed to a larger extent than toner with which the portion thereof except the non-paper passing portion having a relatively low hardness comes into contact is, and hence unevenness occurs in the glossiness of the image after the fixation to reduce the quality of an electrophotographic image formed on the full-size paper in some cases.